Add has_merge_request field to vulnerabilities GQL

What does this MR do and why?

Related to Backend: Add hasMergeRequest filter to Vulnerab... (#371313 - closed) and it is MR-6 to add has_merge_request argument to vulnerabilities resolver.

Screenshots or screen recordings

How to set up and validate locally

1. Clone https://gitlab.com/gitlab-org/govern/threat-insights-demos/issue-390071-verification/ under path root/mr-link-on-branch and run pipeline for branch remediate/test-vulnerability-1-D20230321T163025.
2. Goto the pipeline security tab and click on the first vulnerability and click resolve with MergeRequest button.
3. Then merge branch remediate/test-vulnerability-1-D20230321T163025 into main. Now the vulnerability_reads latest record should have has_merge_request value set for it.
4. In GrahpiQL or Rails console run the below query

{
  project(fullPath: "root/mr-link-on-branch") {
    name
    vulnerabilities(reportType: SAST, hasMergeRequest: true) {
      nodes {
        id
        uuid
        description

      }
    }
  }
}

and we should get output like

{
  "data": {
    "project": {
      "name": "MR Link On Branch",
      "vulnerabilities": {
        "nodes": [
          {
            "id": "gid://gitlab/Vulnerability/797",
            "uuid": "01abf358-28e6-57fc-b89e-a8630898bfe2",
            "description": "Test vulnerability description"
          }
        ]
      }
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #371313 (closed)